Plutonium purification process employing thorium pyrophosphate carrier



United States Patent A U 'PLUTONIUM PURIFICATION PROCESs EMPLOY- ING THORIUM PYROPHOSPHATE CARRIER Application February 7, 1949 Serial No. 75,063

4 Claims. (Cl. 23-145) No Drawing.

This invention relates to the separation and purification of plutonium and particularly to the separation of plutonium from radioactive elements of lower atomic weight.

In this specification and claims, the name of the element designates generically the element both in its free state and combined in a compound. The element in its free state is designated by the term elemental or by its specific state, such as metallic."

An object of the present invention is to provide a method for separating plutonium from certain elements with which it is contaminated as a result of its formation by the neutron irradiation of uranium.

Additional objects and advantages of this invention will be evident from the following description.

It-has been found that plutonium ions in the tetravalent state may be separated from an aqueous solution by precipitating the tetravalent plutonium ions with pyrophosphate anions. It has further been found that tetravalent plutonium ions contained in an aqueous solution in a concentration so low that said plutonium ions will 1 not be precipitated directly as the pyrophosphate may be removed substantially quantitatively from the solution with a carrier precipitate of certain pyrophosphates which are insoluble in aqueous solutions. The pyrophosphates which have been found to be most useful as plutonium carriers include the uranous, ceric, zirconium, and particularly thorium pyrophosphates. These carrier pyrophosphates, for example thorium pyrophosphate, have been found to carry tetravalent plutonium ion quantitatively but do not carry the trivalent or hexavalent plutonium ion, the trivalent rare earth ions or the uranyl ion from aqueous acidic solutions. The process of this invention is based upon the foregoing discoveries. Broadly, this process comprises forming an aqueous acidic solution of plutonium ions in the tetravalent state and elements with which it is normally contaminated in neutron irradiated uranium, treating this solution with a soluble thorium compound and a soluble pyrophosphate compound whereby a carrier precipitate of thorium pyrophosphate is formed in said solution, separating the thorium pyrophosphate precipitate together with the tetravalent plutonium carried with said precipitate, dissolving said precipitate, reducing the tetravalent plutonium ions contained in the new solution to the trivalent state, and re-precipitating the thorium pyrophosphate, leaving the trivalent plutonium ions in. the solution in a substantially pure state.

This process is not only useful in obtaining plutonium in an uncontaminated state but is also useful in obtaining radioactive fission products. The usefulness of these radioactive fission products as tracers and as therapeutic agents is well known.

The plutonium may be placed in solution by dissolving a neutron irradiated uranium mass in an aqueous solution of a strong inorganic acid or a highly ionized organic acid, which does not form complexes with heavy metal ions. In general, nitric acid and hydrochloric acid r- I 2,884,30 Patented Ap 1959 are to be preferred, although sulfuric acid may be used if consideration is given to the tendency of sulfuric acid to form plutonium complexes. It will also be apparent that certain acids are excluded depending upon the choice of cation for the carrier precipitate. Thus, if thorium pyrophosphate were to be used as a carrier, an acid would not be chosen which would form an insoluble compound with thorium. Other considerations, such as the formation of complex ions may exclude other acids in certain instances. However, nitric acid and hydrochloric acid are highly satisfactory in all cases and it is unnecessary to attempt to use other acids for the adjustment of the pH of the solution. After the plutonium is brought into solution, the acidity of the solution is adjusted within broad limits, for example from 0.5 to 5 N, but preferably between about 0.5 and 2 N. It is essential that the solution be acid prior to the precipitation step if the plutonium is to be separated from trivalent rare earth contaminants by the precipitation since the trivalent rare earth pyrophosphates are insoluble in alkaline or neutral solution.- The precipitate should not be permitted to remain in contact with acid solutions for long periods of time since the precipitate may be hydrolyzed, but this eflt'ect is very slow at room temperature.

It is important that the plutonium be present in the solution in the tetravalent state since it has been found that only the tetravalent plutonium ion forms an insoluble pyrophosphate in acid solutions. Relatively stable states of plutonium in the ionic state in aqueous solution are the +3, +4, and the +6 states. The normal state of plutonium when it is placed in solution by the dissolving of neutron irradiated uranium in nitric acid or hydrochloric acid is the +4 state, so that if the solution is formed in this manner, ordinarily, no further precautions need be taken to insure that the plutonium is in solution in the tetravalent state. However, should the plutonium be placed in solution by any other method which would make the valence state of the plutonium ions questionable, the plutonium may be treated with suitable oxidizing or reducing agents, as the case may be, to insure that it is in the tetravalent state. The oxidation-reduction potentials (referred to the normal hydrogen electrode) linking Pu, Pu, and Pu in 1 1.04 volts By referring to a table of oxidation and reduction agents with their respective potentials, such as given by Latimer and Hildebrand in the Handbook of Chemistry and Physics, suitable agents may be chosen to treat the plutonium contained in solution to insure that it is present in the tetravalent state.

The removal of the tetravalent plutonium from the solution with a carrier may be effected by the precipitation of the carrier in the solution, by the treatment of the solution containing plutonium with the preformed carrier, or by passing the solution through a bed of the carrier. However, it has been found preferable to form the carrier in the solution. This may be done by introducing a soluble compound containing the cation, for example thorium chloride or thorium nitrate where thorium pyrophosphate is to be the carrier, into the solution. The carrier precipitate may then be formed by introducing a soluble pyrophosphate compound, such as potassium or sodium pyrophosphate or pyrophosphoric acid into the solution. The order or method of contacting the plutonium-containing solution with the carrier is not essential and variations may be introduced Th'epyrophosphate'"carriers herein described "are "soluble in an excess of pyrophosphate, presumably by the formation of pyrophosphate complexes, such as thorium pyrophosphatefcomplexf Although a'p'recip'itate oftthorium pyrophosphate canirbe 'formed where the .m'olarity'of' the pyrophosphate in fsolution"is"te'n times 'as great the ifiolalrityofthe thorium; it' is 'desirable to form'the car rierpyrophosphate inasolution'where"the mola'rities of the-"*cationiandofthe pyrophosphate do nofdifir 'too greatlyffronrstoichiometric proportions. It.is, of course, desirable; jto maintain the concentration of free "pyrophbs'phate "io'ngin" solution at a "minimum during the 'pre-' cipi tat ion'step'. t Ithas'also been found desirable to carry out"the carrierprecipitation"at room temperature particitla rly the"'solution contains nitric acid, since the nitiie 'acid solutionmay oxidize thePu+ ions at elevated temperatures:'-- Precipitation is ausually complete" with relativelyishortperiods ot digestion even at roomtem perafture." Theithoriurn".pyrophosphate-plutonium' pyrm pltb's'phateiprecipitate is'"a white crystalline precipitate itimayflpbe separated :fromthe solution by filtration, deean tion; or preferably ,centrifugation;

T step offprecipitating 'tetravalent' plutonium with a p hosphateicarrier maybe used to'separate pluirOm'Cni'oSt elements "with which it"is' contamina'ted' except-those which' arenormally tetravalent in aqueous acidic "solution. Thus; this "step will separate plutoniumfrom the rare earths (including cerium, which will normallybepresent aqueous solution in the trivalent state),'jRb, Sr', Y, Cs, Ba; Kr, "and Xe. This step mayfalsobeused to:separate' plutonium from uranium bec'ause'jbyj fllnormal process of dissolving neutron irradiated uianiumiin nitric. acid, the uranium is present in' the uranyl nitrate hexahydrate thus formed, in the hexavalent'statein which state it is not carried with the ThP O -i PnPgO-fprecipitate:* This step of the process i 'soibe used"as"'an'analytical procedure 'to deter- ""e' -helr'elative' amounts of trivalent, tetravalent, and hexalvalentplutonium'in solution, since the tetravalent plutoniumj'wilfbe carried'with-a pyrophosphate precipitateiiwh 'lethe trivalent and hexavalent plutonium will remain solution.

Followinglfl e Separation of the ThPO- PuP O-, precipitate,"the precipitate is 'placedin solution. This may be accomplished by a treatment of the precipitate with an excess 'of 'pyrophosphate ion. For example, the thorium-plutonium pyrophosphate may be introduced into a pyrophosphoric acid solution or into" 'a solution of an alkali metal pyrophosphatewherein the free pyrophosphate; ion "is in' considerable excess with respect to the combined amount oflplutonium and thorium pyrophosphatesr Following-the dissolution ofthe carrier precipitate,,.the. .plutoninmlis, reduced to the trivalent. state. Th hoice of. .redu,cing agents will. depend to some .extent upon the concentrations. of the reactants, and upon the eflect oi the iiet'lucing agent andproducts thereof upon tlie sdlubilitieskof thetions. present. As to .the potentials required'of the reducing agent, it has been found that where the ,conc entrationof the pyroph-osphate ion is-in the vicinity of Oil m, a reductant having a potential more positivethan. about 0.8- volt reduce the tetravalent plutonium presentin theplutonium pyrophosphate complexavto the trivalenttstatet .U'nder any conditions of concentrationr normally,encounteredfin this process, reducing agents havinga potential more positive than. about 0.f l; vo1t,wi1l accomplish the reduction of the tetravalent plutonium.. The effect. of. thereductant and. reduction products uponthe solubilities of ions. present mayreadily be detem inedmby. consulting. standard "tables" of solubilities;. Representative examples of reducing agents which hauetbeen. .found .to .besuitable are .hydroxylamine, hy- (llfQgEIMPCFOXidB, and .oxalate. ions. Following. the reductron of ttheaplutonium to ,.the. trivalent. state the ions. of the. original carrier. are .again precipitated from. the solucarrier "cation" into the solution 'so "that" the ratio of car-" rier cation to pyrophosphate anion approaches st-oichiometric proportions. The precipitate thus formed is then separated from the solution by any suitable method. This step not only frees the-plutonium from the carrier but is also effective in increasing the decontamination of the plutonium. For example, zirconium, which ionizes'only in the 'tetravalentstate and'which maybe'carried by the original carrier, will be removed-from the "solution..con-' taining plutonium by thissecond-pl ecipitation step-v, Thus, this method proves an effective-:separatiom process-for separating plutonium from zirconium. Even fission products having "tetravalent iand lower oxidation states may be separated from plutonium "by this step of the process by the suitableir'selectionof a 're'ducing agent so that the plutonium but not the fission product is reduced from the tetravalent state. Thus, in the final step plutoniumwill-be-redliced-to the non-carri'able-state "with pyrophosphate-but the fission 'pr'odiictwill- 'not' be reduced and-WilP-be carried fromthe'solution -withthe pyrophosphate carrier. Decontamination of plutonium by this process may be 'increas'ed indefinitely by performing this process as a cycle-andrepeating'thecycle. Following the re-precipitationof the'carrier precipitate," the plutonium may 'be'oxidized-to the-tetravalenbstate, thus completing a cycle and the entir'e'cycle may-"be carriedout as often as desired. p

This invention will befui'ther-illustrated by the following specific example.

micrograms of LaClg'weredisso'lvedin "1 M HCl. Tracer quantities of Pu(NO 3) and Pu(N0) were "introduced into'this solution. Twohundred"sixty=six'micrograms of Na' P O werethen'intro'duced into an'aliquot of the above solution having "a finalvolum'e of 0.12 ml." A white crystalline precipitate "formed and this was digested at roomtemperature' with stirring "for eight minutes. This precipitate was then separated from the solution by centrifugation; The'solution andtheprecipitate were analyzed by radiometric m'eth'ods'andit was" found that 98.4%"015 the tetraval'entplutonium'had carried with the "'ThP O More than'99 %"'of" the trivalent plutonium and more than'99% of" the lanthanum" remained in the solution. Following the 'separation'of the'ThP O plutonium carrier precipitate fromthe solution "containing the lanthanum and trivalent plutonium,- the precipitate is dissolved'by introducing the precipitate into 0.15 'ml. of 0.1' M NagPgO andidi'gesting 'at60 for'approximately 'one hour. Following the dissolution'of the precipitateithe' tetravalent."plutonium 'isreduced to the trivalent state"by treatm'ent' with hydroxylamine hydrochloride. The solutionis'then'contacted with 2800 micrograms of Th+ (as the chloride), thus causing 'a precipitate of ThP 'o to' form. This. precipitate is digested for ten -mini1tes at room temperature with stirring and then separated from the solution, leaving the. trivalent plutonium in solution; Analysis" of the precipitate discloses that less than 2% of-the plutonium carries with the precipitate.

it is to be understood,"ofcourse, that the above example is merely illustrative and does'not limit'the scope of this invention. Other mixtures of plutonium with elements of lower atomic numbers may be employed as starting materials and other acids'and other cations may be substituted for those of 'the.'above"example,' within the scope of the foregoing'de'scription' Likewise, the process of the example maybe modified byincoiporating other purification steps in addition to theuse ofthe steps previously described. In general, it may be said that the useof anyequival'ents or modifications of procedure which'would naturally occur to'one skilled in the art'is included in"the-scope 'ofFthis'iinvention. Only such limitations should be imposed on the scope of this invention as are indicated in the appended claims.

What is claimed is:

l. A method of purifying plutonium, which comprises adding a soluble thorium compound and a soluble pyrophosphate compound to an aqueous acidic solution of tetravalent plutonium, separating the ThP2OqPuP2O7 precipitate thus formed from the solution, dissolving said precipitate, adding to the solution thus formed a reducing agent having a potential more positive than about O.8 volt, whereby the tetravalent plutonium is converted to the trivalent state, precipitating ThP O- in the resultant solution and separating said ThPgOq from the solution of trivalent plutonium.

2. A method of separating plutonium from elements with which it is normally contaminated in neutron irradiated uranium, which comprises forming an aqueous acidic solution containing said plutonium in the tetravalent state and contaminants, adding a soluble thorium compound and a soluble pyrophosphate compound to said solution, the pyrophosphate compound being approximately stoichiometrically equal to the thorium and other tetravalent ions in said solution, separating the ThP207PuP2O7 precipitate thus formed from the solution, dissolving said precipitate, adding to the solution thus formed a reducing agent having a potential more positive than about 0.8 volt, whereby the plutonium is converted to the trivalent state, precipitating the ThP O and separating said ThP O precipitate from the solution containing the trivalent plutonium.

3. A method of separating plutonium from zirconium, which comprises forming an aqueous acidic solution containing said plutonium in the tetravalent state and zirconium, adding a soluble thorium compound and a soluble pyrophosphate compound to said solution, the amount of said pyrophosphate compound being approximately stoichiometrically equal to the combined amount of thorium, plutonium, and zirconium ion present in the solution, separating the ThP2O7-PuP2OqZI'P2O7 precipitate thus formed from the solution, dissolving said precipitate, adding to the solution thus formed a reducing agent having a potential more positive than about 0.8 volt, whereby the plutonium is converted to the trivalent state, precipitating ThP O- -ZrP O- and separating said precipitate from the solution containing the trivalent plutonium.

4. A method of separating plutonium from radioactive fission products, which comprises forming an aqueous solution containing said plutonium in the tetravalent state and fission products, adjusting the acidity of said solution to between 0.5 and 2 M, dissolving in said solution a soluble thorium compound, introducing Na P O into the solution in amount approximately stoichiometrically equal to the plutonium and thorium ions present in said solution, separating the ThP207-PuP207 precipitate thus formed from the solution, introducing said precipitate into a pyrophosphate solution wherein pyrophosphate is in sufficient concentration to dissolve the precipitate, adding to the solution thus formed hydroxylamine whereby the plutonium is converted to the trivalent state, introducing sufiicient additional soluble thorium compound in the solution to precipitate the thorium ion in the solution as ThP O and separating said precipitate from the solution containing the trivalent plutonium.

No references cited. 

1. A METHOD OF PURIFYING PLUTONIUM, WHICH COMPRISES ADDING A SOLUBLE THORIUM COMPOUND AND A SOLUBLE PYROPHOSPHATE COMPOUND TO AN AQUEOUS ACIDIC SOLUTION OF TETRAVALENT PLUTONUIM, SEPARATING THE THP207-PUP2O7 PRECIPITATE THUS FORMED THE SOLUTION, DISSOLVING SAID PRECIPITATE, ADDING TO THE SOLUTION THUS FORMED A REDUCING AGENT HAVING A POTENTIAL MORE POSITIVE THAN ABOUT -0.8 VOLT, WHEREBY THE TETRAVALENT PLUTONIUM IS CONVERTED TO THE TRIVALENT STATE, PRECIPITATING THP2O7 IN THE RESULTANT SOLUTION AND SEPARATING SAID THP2O7 FROM THE SOLUTION OF TRIVALENT PLUTONIUM. 